1. Field of the Invention
This invention pertains to the general field of packages and packaging methods for semiconductor devices; in particular, the invention relates to a lead frame design that permits the trim and form operations to be carried out without interference by the mechanical stresses created during the die bonding and molding steps.
2. Description Of the Prior Art
The process of manufacturing semiconductor devices involves several steps whereby an integrated circuit chip is bonded to a foil-type lead frame and encapsulated in epoxy or other molded resin. The lead frame consists of a plurality of leads etched or stamped out of a thin metallic-foil tape, and the inner ends of the leads are usually bonded to the solder bumps of the integrated circuit chip by a thermal compression method. The chip is then encapsulated in plastic by an injection molding process that results in a chip package having the outer ends of the leads exposed to the outside of the integrated circuit for connection to a circuit board. In a typical prior art system for bonding and encapsulating integrated circuits, a plurality of such circuits is linearly arranged in a single workpiece, which contains a series of identical lead frames etched or stamped on a strip of metallic substrate. A chip is bonded to each of these premanufactured lead frames and they are loaded into a bottom mold with the number of lead frames in a single batch being determined by the mold size and the capacity of the particular molding equipment being used. A top mold is moved into place atop the bottom mold and some means is provided within the molding equipment to heat the molds to the proper temperature for molding, and to subsequently cool them for curing purposes. The heated molds are clamped together by the molding equipment and when the temperature is right, usually at approximately 175.degree. C., molding material, such as epoxy in pellet form, is placed in the mold set through bores formed in the top mold. Plungers are then inserted into the bores of the top mold and a pressurizing force is exerted on the plungers. The combination of the plunger force and the temperature causes the epoxy pellets to liquify and flow into the cavities provided in the mold set, which determine the configuration and location of the molded plastic that encapsulates the integrated circuits. Upon completion of this step, the mold set is cooled to induce curing of the epoxy; then the plungers are pulled from the mold set, the mold set is unclamped, and the top mold is lifted from the bottom mold. The lead frames are then removed from the bottom mold and carried to further equipment for trimming and forming, and otherwise operating on them to finish fabrication of the electronic circuit packages. Typically, the outer leads of each package need to be bent (formed) to conform to the requirements of the printed circuit board for which they are designed.
In order to position each lead-frame workpiece correctly within each work station performing the various steps, each lead frame contains alignment holes that match corresponding alignment pins in the supporting equipment, with which they become engaged for proper positioning at each station. These same holes are used, as the workpiece progresses through the assembly line, during the steps of bonding, molding, trimming and forming. Since each step is carried out within very narrow tolerances, it is extremely important that these alignment holes remain in constant spatial relationship with respect to the rest of the lead frame.
Unfortunately, the molding step subjects the lead frame to structural stresses caused by difference in the heat expansion coefficients of the metal and the plastic material. Thus, during the cooling phase, the encapsulated lead frame is stressed by the cooling plastic and is subjected to bending, waving and twisting of the leads. This deformation often results in a misalignment of the alignment holes in the lead frame with the receiving pins in successive work stations, which causes a mismatch between the holes and the pins and considerable quality control problems and waste. Therefore, there exists a need for an improved design of the alignment holes that results in a stress-free lead frame.